In recent years, energy saving has been considered to be an important technical issue even in electrophotographic devices, and a drastic reduction in the amount of heat required by a fixing apparatus has been mentioned as one way to achieve this. Accordingly, a need for so-called “low-temperature fixability” in a toner, in which fixing with lower energy is possible, has been increasingly raised.
Conventionally, a technique involving increasing the sharp melt properties of a binder resin is known as an effective method to enable fixing to be performed at lower temperatures. In terms of this, polyester resins exhibit excellent characteristics.
On the other hand, from the separate viewpoint of improving image quality, reduction of the toner particle size and sharpening of the toner particle size distribution have progressed for the purpose of attaining higher resolution and higher definition. In addition, a spherical toner is now suitably used for the purpose of improving transfer efficiency and fluidity. As a method for efficiently preparing spherical toner particles with small particle sizes, a wet method is now used.
As a wet method capable of using a sharp-melting polyester resin, a “solution suspension” method has been proposed in which spherical toner particles are produced by dissolving a resin component in an organic solvent which is immiscible with water, and dispersing the resultant solution in an aqueous phase to thereby form oil droplets (see Japanese Patent Application Laid-Open No. H08-248680). According to this method, a spherical toner with a small particle size can be easily obtained in which polyester having excellent low-temperature fixability is used as a binder resin.
Further, for the toner particles produced by the above-described solution suspension method using polyester as a binder resin, capsule type toner particles have also been proposed for the purpose of attaining even further low-temperature fixability.
Japanese Patent Application Laid-Open No. H05-297622 proposes the following method.
An oil phase is prepared by dissolving and dispersing a polyester resin, a low-molecular-weight compound having an isocyanate group and other components in ethyl acetate, and droplets are prepared in water. Thereby, the compound having an isocyanate group is caused to undergo interfacial polymerization at the droplet interfaces to thereby produce capsule toner particles having polyurethane or polyurea as an outermost shell.
In addition, Japanese Patent Application Laid-Open No. 2004-226572 and Japanese Patent Application Laid-Open No. 2004-271919 propose methods in which toner base particles are prepared by a solution suspension method in the presence of resin fine particles formed from any one, or a combination of two or more, of a vinyl resin, polyurethane resin, epoxy resin and polyester resin, to thereby prepare toner particles each having a toner base particle surface covered with the above-described resin fine particles.
Japanese Patent No. 3,455,523 proposes toner particles obtained by a solution suspension method using urethane-modified polyester resin fine particles as a dispersant.
International Application No. WO 2005/073287 proposes core-shell type toner particles formed by a shell layer (P) having one or more film-like layers formed from a polyurethane resin (a) and a core layer (Q) having one layer formed from a resin (b).
In these core-shell type toner particles, the viscosity of the core portion is lowered and the poor heat-resistant storage stability of the core portion is compensated with the heat-resistant storage stability of a shell portion. In this case, since a substance which is relatively strong against heat is used as the shell part, it is necessary to highly cross-link the substance or to use a substance having a high molecular weight. Consequently, there is a tendency for low-temperature fixability to be inhibited.
On the other hand, if the addition amount of the coloring material in the toner is increased, the coloring power of the toner is increased and the consumption of the toner is decreased by controlling the dispersion state of a colorant. By decreasing the consumption of the toner, high-quality line and character images with reduced scattering can be provided. Further, on a sheet of paper, uneven portions are reduced and gloss is uniformized. Moreover, due to the decreased consumption of the toner, the toner container and the electrophotographic apparatus can be miniaturized. In addition, running costs can be reduced, and power consumption can be decreased.
However, if the added amount of the coloring material in the toner is simply increased, due to dispersion defects, for a color toner, the color gamut is liable to narrow, and fixing inhibition tends to occur as a result of an increase in toner hardness owing to filler effect. Further, a large amount of colorant comes out of the toner particle surface and tends to contaminate the carrier part and sleeve part for a two-component developer. Moreover, when the moisture absorption properties of the colorant on the surface are high, a difference in charge amount tends to occur depending on environments.
While these effects are noticeable for a black toner using carbon black, even when a yellow toner is used, these effects tend to occur if the addition amount of the coloring material is increased. Therefore, there is a need for improving the technology of dispersing a colorant for each color.